The injectors in a said combustion engine are controlled with the help of an engine control device, which controls the opening times for each individual injector in order thus to control the amount of fuel to be injected into the cylinder belonging to the injector. Here, apart from the duration of the opening time, the prevailing fuel pressure in the fuel accumulator and the injector's opening degree determine how large a fuel amount is injected into the cylinder. In order to keep emissions of contaminants, re-suiting from the combustion in the respective cylinder, at a minimal level, it is important to know exactly how large an amount of fuel is injected into the cylinder, so that the desired mixture of air and fuel to achieve said minimal emission level may be achieved. The fuel pressure in the fuel accumulator and the opening time of the respective injector are easy to determine, but the injector's features and, consequently, its opening degree, may change over time. This fact is not critical as long as the opening degree may be determined with precision, since e.g. a reduced opening degree may be compensated by an extended opening time of the injector. Opening degree as used herein therefore means how much the injector is open when it is opened, i.e. it is a measure that determines how much fuel leaves the injector per time unit at a given fuel pressure in the fuel accumulator when the injector is open. If one wishes to inject a certain amount of fuel, the opening degree at a certain fuel pressure is thus decisive for the duration of the injection period, which is sometimes called the injector's injection duration. Over time, the valve of the injector may be increasingly clogged, so that the opening time must be extended in order for a certain fuel amount to be injected. However, parts of the injector's valve may also age or be compressed over time, so that more fuel is injected per time unit than expected, i.e. the injector's opening degree increases with time. In this case, the injector's opening time for a given fuel amount should be shortened. This way of controlling the opening time may be called fuel amount adaptation. However, from an emission point of view, it is most important to know the exact opening degree of the respective injector at a given point in time, in order to know the exact fuel amount injected into the cylinder.
US 201210150417 A1 describes a method of the type defined above, which tests whether an injector of the combustion engine is totally or partly clogged by carrying out a test injection of fuel into the associated cylinder, while simultaneously a change of the fuel pressure in the fuel accumulator during the test injection is measured in order to thus obtain a measure of the injector's opening degree.